The typical hopper bottom bin for flowable bulk solid materials, such as grain, metal ores, or plastic pellets, has a vertical cylindrical section joined at its lower edge to a conical or frustro-conical hopper. The bin is filled through an inlet opening at the top of the cylindrical section, and is emptied through an outlet at the lowermost point of the hopper. Discharge apparatus for guiding the material from the bin to its destination or starting and stopping the discharge flow is commonly bolted or welded to a bolt ring mounted around the outer surface of the hopper at its bottom.
Although the above-described configuration is typical, the geometry of bins varies. For example, the inlet opening may be centered over the bin, or positioned to one side of the bin roof or on a side wall of the bin. The hopper may be a right circular cone with a centered outlet, or a cone having an oblique axis and an outlet which does not lie along the central axis of the bin. Other variations, for example in the cross-sectional shape of the bin, are also found.
A problem occuring in nearly all bins, despite these variations, is the segregation of material according to particle size, shape or density as it is introduced into a bin. Material deposited in a bin generally forms a conical pile centered under the inlet opening, with coarse particles tending to roll outward and down to the periphery of the bin and fine particles tending to accumulate in the center. This results in segregation of different sized particles in different regions within the bin.
Another problem common to many bins is a tendency for segregation of material to become enhanced as the material is discharged from the bin. This is a result of funnel flow, where material directly above the discharge outlet moves downward at a greater speed than material elsewhere, while material in some regions of the bin may not move at all. If the outlet is located directly under the inlet, the fine particles which tend to accumulate directly under the inlet will be discharged before the coarser ones, resulting in more pronounced segregation. If the outlet is elsewhere, the coarser particles will be discharged first, and a more pronounced segregation will still result.
A further undesirable effect of funnel flow is that it causes layers of material deposited at successive time intervals to intermix in an uncontrolled manner. In some circumstances, it is desirable to have material exit a bin in the same order that it entered; in other situations, it may be desirable for material from successive layers to be blended together as the bin is emptied. Adequate control of the extent of the intermixing of layers, either to prevent or to promote their blending, is not provided by the structure of most bins.
In contrast to funnel flow, where some material in a bin moves downward while a portion remains stationary, "mass flow" is a name given to a condition where all material in the bin moves simultaneously, and none stands still. "Laminar mass flow" designates a special case of mass flow, where all material moves in the same direction at the same speed, and no cross-movement of material occurs. Thus, all particles of material remain at the same position relative to each other, within a mass which moves downward as a unit.
Prior art devices developed in an attempt to control segregation and blending of material have been costly to manufacture and install. For example, a multiple-opening bin bottom made up of multiple adjacent hoppers has been proposed to provide laminar mass flow in a bin for the withdrawal of material in the order of its entry into the bin. But, the complicated design of this apparatus makes its fabrication expensive. Furthermore, such an apparatus cannot be retrofitted to an existing bin without a costly restructuring of the bin bottom.
Another device designed to provide laminar flow of material and minimize enhancement of segregation during its discharge includes a large cone positioned within a hopper and extending from the bottom to the top of the hopper. Vanes mounted on the cone extend out to the hopper walls. Because the device is shipped in completed form, with the cone and vanes mounted in a hopper at the factory, the device is cumbersome and difficult to ship. Furthermore, this device requires large amounts of material for its manufacture.
A blending apparatus disclosed in U.S. Pat. No. 4,286,883 to Johanson includes a conical insert to promote mass flow movement of material in a self-emptying hopper. A complicated and bulky structure to suspend the cone within the bin and a special hopper design are required, making this apparatus expensive to manufacture, and difficult to scale up to larger bins.
Thus, the bulk materials handling art has long needed a system which can achieve or exceed the flow control provided by prior art devices, while being easily retrofitted to a variety of existing hopper bottom bins, and selectively fine tuned to achieve the desired flow within a particular bin. Ideally, such a device would be inexpensive to manufacture and ship, and would be installed with little or no on-site alteration of pre-existing bin structure being necessary.